Avoid getting sleep at all costs. Studies have shown that significant sleep deficit results in mental impairment similar to intoxication along with a decline in motor skills.
Eat a lot of junk food with high fat and sugar content, and avoid fruits, vegetables, fish, and other "brain" foods.
Never Exercise, otherwise you will experience the benefits of better blood flow to the brain along with improved Neuronal metabolism.
Don't try and learn a new skill or challenge yourself in anyway, or you might stimulate the growth of new neural connections.
Avoid difficult games, problems, and puzzles - that way your I.Q. will stay the same or decline over time.
Accept the status quo at work and in life to keep the creative side of your brain dormant.
Abuse drugs, alcohol, or other substances to keep your brain cells suppressed, and eventually damage them permanently.
Let your emotions run wild - especially anger. Such highly charged states are not conducive to good mental function.
Never look for more than one solution to a problem - that way you won't have to engage too many neurons in the task at hand.
If you follow this advice you will experience the down side of KISS (Keep It Simple Stupid) - something smart people say when they are over thinking a problem, except in your case you will actually be stupid and simple :-).